The invention relates to a pneumatic brake booster. Such a brake booster designed as a vacuum brake booster for a motor vehicle is known from WO 95/01272, which is incorporated by reference herein.
Pneumatic brake boosters normally have a fixed force transmission ratio, i.e. an actuating force introduced by means of an input element into the brake booster is transmitted from a smaller area associated with the input element to a larger area associated with the power output element, mostly via a so-called reaction disk which is made of elastomeric material and behaves like a fluid. Under certain circumstances, e.g. in an emergency braking situation, it is however desirable to provide a user with as high a braking force boost as possible so that, starting from a specific input force, as high a brake pressure as possible may be generated.
In the said WO 95/01272 it is therefore proposed that the smaller area, which acts upon the reaction disk and is associated with the input element, be further reduced in size after a specific input force is exceeded. Said reduced area, compared to the non-reduced area, given the same input or actuating force penetrates further into the elastic reaction disk, with the result that the control valve of the brake booster opens correspondingly wider, resulting in a correspondingly higher differential pressure in the brake booster and hence in an increased force boost. The solutions proposed in WO 95/01272 are however of a relatively complex construction and moreover make assembly of the brake booster considerably more complicated.
The object of the invention is therefore to provide a pneumatic brake booster with a device for varying the force transmission ratio in dependence upon the force exerted upon the force input element, which device is to be of as simple a construction as possible and, above all, easy to assemble.
Proceeding from a brake booster of the type described, said object is achieved according to the invention in that the device for varying the force transmission ratio is designed as a preassembly unit in the form of a cartridge, which after preassembly is insertable into the control valve. The cartridge comprises a plunger, of which the end facing the reaction disk is connected to a piston, a hollow-cylindrical operating piston, which surrounds the previously mentioned piston and is displaceable relative to the piston, a stop, which is provided on the plunger and defines for the operating piston an initial position, in which the areas of operating piston and piston facing the reaction disk form a common, flush area, and a spring, which is supported against the plunger and which biases the operating piston towards the stop. In said manner a compact, easy-to-preassemble unit is provided, which after preassembly need merely be inserted into a corresponding recess of the control valve. There is no need for more extreme modifications of the structural design of the brake booster or control valve, rather the cartridge used according to the invention may be integrated without a high outlay into many existing brake booster constructions. The possibility therefore exists of being able to offer one and the same brake booster construction with or without variable force transmission.
The operating piston is preferably guided in a slidingly displaceable manner on a stem of the plunger. Such a construction removes the need for a separate guide for the operating piston and results in a compact style of construction.
In order further to simplify the structural design and reduce the size of the device for varying the transmission ratio, in preferred forms of construction of the brake booster according to the invention the plunger stem at its end facing the force input element is connected to a plate, against which the spring, which biases the operating piston towards the stop, is supported. In said case, the outside diameter of the operating piston is advantageously greater than the outside diameter of the plate so that the operating piston, in the event of greater input forces, may be supported against a step, which is situated in the recess provided in the control valve housing for receiving the cartridge. The plate may be formed integrally with the plunger stem.
The stop defining the initial position of the operating piston is preferably formed by a retaining ring (snap ring), which is held in a groove of the plunger stem. Such a construction is inexpensive and easy to assemble.
The spring biasing the operating piston into its initial position may have a linear spring characteristic curve. Equally, however, said spring may have a progressively rising spring characteristic curve, with the result that the braking force boost of the brake booster according to the invention increases progressively, beginning with the movement of the operating piston out of its initial position up to the attainment of an end position of the operating piston. The end position of the operating piston may be defined e.g. by the already mentioned shoulder in the recess of the control valve housing used to receive the cartridge.
The piston, which is connected to the end of the plunger facing the reaction disk, preferably has an at least approximately spherical-segment-shaped protuberance on its area facing the reaction disk. By means of such a protuberance a specific, so-called xe2x80x9centry behaviourxe2x80x9d of the brake booster is realized. To put it more precisely, at the start of an actuation of the brake booster the spherical-segment-shaped protuberance, given a defined force, penetrates more deeply into the reaction disk, with the result that the control valve in the initial phase of a braking operation may open slightly wider and the brake booster therefore relatively quickly provides a specific braking force boost.